The principal objective of this research project is the characterization of the dynamic state of biological membranes. Particular emphasis will be placed on the interrelation between cell surface lateral diffusion and systematic flow, with the ultimate goal of relating these properties to specific intracellular interactions and cell functions. For this purpose, a number of systems, amenable for study, have been identified in which there is clear evidence of the spontaneous and/or ligand-induced redistribution of cell surface receptors. These are: guinea pig sperm, locomoting and spreading fibroblasts, and cell transferrin receptors. In the long term, a better understanding of these systems may have implications to such diverse areas as fertilization, the invasiveness of neoplastic cells, and the resistance to infection. The principle physical techniques to be used in these studies are fluorescence redistribution after photobleaching (FRAP), and fluorescence correlation spectroscopy (FCS). In the FRAP technique, the lateral transport of fluorescently labeled membrane-bound probes is characterized through measurements of the surface distribution as a function of time after an initial, localized or patterned, photobleaching pulse. In FCS, in contrast, molecular motions and cluster sizes are determined through calculations of the space-time correlation functions of spontaneous stochastic fluctuations, without an initial system perturbation.